Treatment of dross for the recovery of zinc



2 Sheets-Sheet l als BY4 'www G. T. MAHLER Filed March 27, 1945 Dec. 30, 1947.

ATTORNEYS G. T. MAHLER TREATMENT OF DROSS FOR THE RECOVERY OF ZINC' Filed March` 27, 1945 2 Sheets-Sheet 2 A, Il No.

Patented Dec. 30,*1947 einen TREATMENT F DROSS FOR THE RECOVERY 0F ZINC George T. Mahler, Palmerton, Pa., assignor to The New Jersey Zinc Company, New York, N. Y., a corporation of New Jersey Application March 27, 1945, Serial No. 585,112

(Cl. 'i5- 24) 7 Claims.

This invention relates to the treatment of dross, and has for its object the provision of an improved method of treating-dross.

Dross, within themeaning of this invention, is any material containing metallic `Zinc in economically recoverable amount andA includes galvanizers dross or hard zinc,gmetal turnings, scrap and the like of zinc base and other zinc-containing alloys, metallicresidues of yzinc rening operations, and the like.V4 The invention contemplates an improved method o f recovering or reclaiming metallic zinc ofcommercial purity from dross. Thus the invention involves introducing the dross at the top of Ya vpacked column externally heated toimaintainameltingzone approximate the top of the columnanda volatilizing zone immediately below the meltingzone,I and-withdrawing zinc vapor from thecolumn at a point approximate the junction of the meltingv andvolatilizing zones. The column preferably includes briquets of carbonaceous material, suchgascoked fuel briquets, and mayconsist wholly of Asuch briquets or of a mixture of such briquets andbriquets of an inert materialsuch as `zsiliconcarbide. Where carbonaceous briquets are included in the column, air may .be admittedv near the bottom of the column in amount suiiicient to convert-some part but not all ofthe carbon contentof these briquets to carbon monoxide. The 'packedcolumn may progressively descend-by. gravity through an externally heated vertical retoraby withdrawingy from the bottom and introducing at the top of the column substantia'ly equivalent amounts of the packing composing the column.l

4The foregoing andA othernovel features of the invention .will be ,betterunderstood from the following description taken in conjunction with the accompanying drawings, in Which,

Eig. 1 is a .sectional elevation of an apparatus particularly adapted for practicing the method of the invention, c y

Fig. 2, is a sectional elevation taken on the section line v2,--2 of Fig. 1,

Fig. 3 is a sectional plan of the condenser taken on the section line 3 3 of Fig. 1, and

Fig. 4 is a sectional plan of the externally heated retort furnace taken on the section line of Fig. 2.

The apparatus illustrated in the drawings comprises a rectangularV .vertical `retort ID suitably mounted within and spaced from a main furnace structure consisting generally of an interior wall II of heat-resistant material, such as lire brick, and an outer Wall I2 of heat-insulating material, such as insulating brick. ,The retort I0 may advantageously be built up of silicon .carbide brick or equivalent heat and zinc vapor resistant material, and extends upward'y beyond the furnace structure II-,I2 through and suitably spaced from an auxiliary furnace Vstructure consisting generally of an interior wall I3 of heat-resistant brick or the like and an exterior 4wall I4 of heatinsulating` brick or the like. Below the furnace structure II-I2, the retort I0 extends into a discharge pit I5, andv is provided at its lower end with discharge doors I6. A valved pipe I'I permits the introduction of air into thelower end of the retort IIJ, somewhat above the discharge doors I6 but below the furnace structure I I I 2.

Just above the main furnace structure II-IZ, but well below the top of the retort I0, a zinc vapor outlet pipe I8 communicates with the retort. The outer or discharge end of the pipe I8 is connected to a condenser I9 through a pipe 2li. The condenser-is generally ofthe type described in the United States patent of E. H. Bunce, No. 1,873,861, and'comprisesan elongated condensing chamber 2l of rectangular section built up of silicon carbide brick or the like. Spaced baiiles 22 depend from the top orV roof o1 the condensing chamber to within a short distance of the bottom and extend alternately from opposite sides of the chamber to within a short distance of the other side. Thev condensing chamber 2I has an upright vapor inlet ue y23 at one end communicating with the pipe 20, and a stack 24 at the other end for exhaust gases. The flue 23 has a normally closed Clean out opening 25, and the stack 24 has a damper 26 for controlling the ilow therethrough of the exhaust gases. The condensing chamber has a tap hole 2'I for molten metal, at the same end asthe stack 24.

Ports 28 for fuel --burners extend through the opposite end walls of the main furnace structure. Three such ports are provided in each end wall and are so positionedthat the gaseous products of combustion from the burners are directed in opposite directions against the opposite side walls of the retort (Fig. 4). Suitable holes 29 extend through the oppositeA side ,walls of themain and auxiliary furnace structures for the accommodation of thermocouples or the like. Vertical ports or nostrils 30 connect the heating chambers of the main and auxiliary furnace structures. The heating chambers of the main and auxiliary furnace structures have exhaust gas fiues 3| and 32, respectively, communicating with a common stack 33, and provided with dempers 34 and 35, respectively. The top of the retort I is normally closed by a removabe cover 3S.

In practicing the invention in the apparatus illustrated in the drawings, the retort I0 is initially lled with appropriate packing to provide a packed column. The packed column may be made up of briquets, or equivalent solid shapes, to impart the porosity customary in such columns. The column may, for example, be composed solely of briquets or agglomerates of carbonaceous material, such for example as the coked fuel agglomerates produced by the method of United States patent of E. H. Bunce No. 1,941,462, or of a mixture of such briquets and briquets of silicon carbide or similar inert material, or solely of briquets of silicon carbide or similar inert material. The briquets or agglomerates may be of any convenient size and shape, the conventional pillow block briquet being well suited for the purpose. In the interest of simplicity, Ihave throughoutV this specification and the appended claims spoken of briquets in a generic sense to include all solid shapes, however formed, of suitable size and shape to provide the contemplated packed col-- umn.

The packed column within the retort I3 is heated (by the burners in the ports 28) until that part of the column below the zinc vapor outlet I8 is at a temperature sufficiently high to Volatilize molten zinc, and that part of the column above the outlet I8 is at a temperature suiliciently high to melt zinc witho-ut volatilization. Dross to be treated isthen introduced on top of the column, and the zinc and other metals in the dross melt and flow or trickle by gravity through the column until they reach that part below the outlet I8, where Zinc is volatilized, and the resulting zinc vapor flows through the outlet I8 into the condenser I9 where it is condensed. Thus, there is maintained in the externally heated packed column, a top melting zone and immediately therebeneath a volatilzing Zone, with the outlet for the resulting zinc Vapor at approximately the junction of the two zones. The top melting zone of the column is heated by the gaseous products of combustion passing through the ports 3!! into the heating chamber of the auxiliary furnace structure. Appropriate regulation of the dampers 3d and 35 controls the amount of heating gases owing through the ports 3Q to maintain the contemplated temperature differential between the melting and volatilizing zones. Thermocouples or other types of pyrometers, suitably positioned in the holes 29, serve to accurately indicate the thermal condition within the heating chambers and hence of the exterior, of the retort I throughout its length. In an apparatus of the construction shown inthe drawings, where the vertical retort I0 is approximately 1 foot by 2 feet in cross section, and the melting Zone of the packed column is 3 to 5 feet in length, the exterior of the retort within the auxiliary furnace structure may be heated to about 1000 C. and the exterior of the retort Within the main furnace structure may be heated to about 1200-1300" C., to provide and maintain the melting and volatilizing zones of the invention. The interior of the retort and packed column do not, of course, attain these temperatures, because of the absorption of heat by the melting of zinc in the melting zone, and by the Volatilization of molten Zinc in the volatilizing Zone.

When normal operating conditions are established, the packed column will be externally heated as hereinbefore explained, and the column will be caused to progressively descend through the retort by withdrawing a suitable amount of the packing material through the discharge doors I6 and introducing an equivalent amount of packing material (including dross to be treated) at the top of the column, the cover 36 being removed temporarily for this purpose. In the apparatus of the drawing, the packing is manually withdrawn from the bottom of the column, but it will be understood that the bottom of the retort may be equipped with a suitable mechanical discharge. All or any desired part of the packing withdrawn from the bottom may be recharged at the top. The dross may be mixed with the packing introduced at the top of the column, or may be separately introduced. Charges oi dross may conveniently be made every 30 or 40 minutes, and the heating of the melting zone of the column is so regulated that substantially all of the zinc and other metals in the dross are melted during this period. As the zinc and other metals in the dross melt in the melting zone, they now into the volatilizing zone and zinc is volatilized.

One common source of dross is the so-called hard zinc from galvanizing operations. When the molten galvanizing bath is sufficiently spent or exhausted. the hard zinc accumulating at the bottom is solidified, usually in a largo lump, and is marketed as such. Hard zinc commonly contains bolts, hooks and similar articles subjected to galvanizing and which become loose and hence lost in the bath. These articles are included in the solidied hard zinc, and are frequently used for handling it. For the practice of the invention, the lumps of hard zinc are broken up into pieces of convenient size for charging into the vertical retort, alone or mixed with fresh packing.

The metallic content of the dross, other than zinc, and of higher boiling temperature than Zinc, as for example iron, copper, lead, tin. etc., remains with the packing and is discharged from the base of the retort, along with the withdrawn packing, in the form of a granular metallic residue. Galvanizers dross customarily contains iron, as well as a heterogeneous mixture of metallic objects such as bolts, hooks and the like. The metallic residue from such iron-containing dross can be readily recovered from the withdrawn packing by magnetic separation. When the dross includes zinc alloys, the alloying metals, such as copper, lead, tin, etc., largely accompany the iron in the metallic residue. and may be selectively recovered from the concentrate of the magnetic separation.

The bottom of the retort I0 is not ordinarily sealed, but merely closed by the doors I6, thus permitting a substantial amount of air to enter. If the doors I6 are sealed, it is advantageous to introduce a controlled amount of air into the bottom of the retort through the pipe I'I. When air is thus admitted or introduced into the bottom of the retort, the packed column should include sufficient carbon, e. g., coked fuel briquets, to convert all of the oxygen of the air to carbon monoxide, and preferably the amount of carbon in the column should be considerably in excess of that requiredtc convert, all of the oxygen in the enteringair Y'to' Ycarbon monoxide. The carbon' monoxide thus formed together with the nitrogen in the air produceadesirabl (113V- draft through the packed column. When the retort is operated without sealing the doors, -t11 e onlyaii introduced is thatcaused, to low'fbygthe stack draft'. Under these conditionsfijjis not necessary-Etc introduce aithrough the 'f pipg` lil. The air'2 `from whatever source, enteringjthebotnitrogendilution of the'rich -zinc vapor,"vvhich may be desirable when producing zinc dust or zinc oxide from the zinc vapor, but is undesirable when condensing the zinc vapor to zinc metal. The up-draft of carbon monoxide and nitrogen does, in any case, prevent the zinc vapor from backing down and condensing on the colder packing or causing bottom fuming.

The presence of carbonaceous briquets in the charge establishes reducing conditions in both the melting and volatilizing zones, so that the formation of skimmings or oxide crusts by oxidation during the melting is minimized, and whatever skimmings may be formed in the melting zone are reduced in the volatilizing zone.

In the apparatus of the drawing, the zinc vapor outlet pipe i8 is inclined upwardly at a sufficient angle to insure downward progress of the packing without objectionably obstructing the pipe. The entrance end of the communicating pipe 253 may be provided with a bale 37 of the louver type to prevent any of the packing entering the condenser. Any appropriate arrangement of baffles can, of course, be used for this purpose. The Zinc vapor is condensed in the chamber 2l of the condenser, and the resulting molten zinc is removed from time to time through the tap hole 21. The zinc metal thus recovered from the dross is usually of intermediate grade, and may be re ned in a rectifying column or columns to obtain Zinc metal of high purity. The zinc vapor from the outlet i8 may be directly (i. e., withoutcondensation) transformed to high purity metal by rectification. Or, the zinc vapor from the outlet I8 may be condensed to zinc dust, or it may be burned to zinc oxide.

Zinc purified from contaminants with boiling points exceeding that of zinc (such as lead) may be obtained by maintaining a section or zone of the retort and packed column immediately below the zinc vapor outlet I3 at such'a temperature that zinc vapor is partially condensed therein but which does not cause the down-owing mol ten dross to freeze. The temperature of this zone of the packed column will be intermediate the temperatures of the melting and volatilizing zones and may be maintained in any suitable manner.

Except for the relatively small amount of carbon that is oxidized to carbon monoxide by air entering or introduced into the bottom of the retort, the packing composing the column is inert. The packing serves to support the dross until it is melted, and to distribute the molten zinc in relatively thin films over the surfaces of the solid shapes of the packing, thereby prometing volatilization. Moreover, the voids of the packed column provide channels for the ready escape of Zinc vapor to the outlet therefor and also for the gaseous up-draft when air is admitted at the base of the column. The packing also serves to conduct heat from the heated wall of the retort to the interior thereof, and silicon carbide and carbonaceous material are particularly ethod of recovering zinc from dross which com esN introducing the drossat the top fa'fpacked4 column, externally heating saidv col'-v urnn torraintain (1) a melting zone near its ytop at a temperature sufficiently high to melt the zinc and tlmetalsin the dross but insucient to volatilize zinc and (2) a volatilizing zone immediately below the melting zone into which the melted dross flows by gravity from the melting zone and zinc is volatilized, and withdrawing zinc vapor from the column at a point approximate the junction of said zones.

2. The method of claim 1 in which the packed column includes briquets of carbonaceous material.

3. The method of claim 1 in which the packed column includes briquets of carbonaceous material and air is admitted near the bottom of the column in amount suiiicient to convert some part but not all of the carbon content of said briquets to carbon monoxide.

4. The method of claim 1 in which the packed column progressively descends by gravity through an externally heated vertical retort by withdrawing from the bottom and introducing at the top of the column substantially equivalent amounts of the packing composing the column. n

5. The method of claim 1 in which the column is composed of a mixture of coked fuel briquets and silicon carbide briquets and progressively descends by gravity through an externally heated vertical retort by withdrawing from the bottom and introducing at the top of the column substantially equivalent amounts of the packing composing the column.

6. The method of recovering zinc from dross which comprises introducing the dross in solid form at the top of a packed column including briquets of carbonaceous material and which progressively descends by gravity through an externally heated vertical retort by withdrawing from the bottom and introducing at the top of the column substantially equivalent amounts of the packing composing the column, said retort being heated to maintain (1) a melting zone approximate the top of the column and (2) a volatilizing zone immediately below the melting Zone into which molten zinc ows by gravity from the melting Zone and is volatilized, admitting air near the bottom of said column in amount sufcient to convert some part but not all of the carbon content of said briquets to carbon monoxide, and withdrawing zinc vapor from the column at a point approximate the junction of said zones.

7. The method of treating dross containing zinc and iron which comprises introducing the dross in solid form at the top of a packed column including briquets of carbonaceous material and which progressively descends by gravity through an externally heated vertical retort by withdrawing from the bottom and introducing at the top of the column substantially equivalent amounts of the packing composing the co1- umn, said retort being heated to maintain (1) a melting zone approximate the top of the column and (2) a volatilizing zone immediately below the melting zone into which molten zinc flows s,4ss,'e15 7 S by gravity from the melting zone and is vola.- REFERENCES CITED timed afmtng air nea? the bottom of said l The following references are vof record in the column m amount sufcient to convert some me of this patent. part but not all of the carbon content of said briquets to carbon monoxide, withdrawing zinc Va- 5 UNITED STATES PATENTS por from the column at a point approximate the Number Name Date junction 0f said zones, and recovering by mag- 1,526,910 Job Feb. 17' 1925 netio separation the residual metallic content of 1,749,127 Bunce et a1 Mar. 4, 1930 the packing withdrawn from the bottom of the 1,754,845 Baly et a] Apr, 15, 1930 column. 10 2,215,961 Hawk Sept. 24, 1940 GEORGE T- MAKLER 1,994,356 Pierce et al Mar. 12, 1935 

